Journal of Electromyography and Kinesiology 24 (2014) 888–894
Contents lists available at ScienceDirect
Journal of Electromyography and Kinesiology journal homepage: www.elsevier.com/locate/jelekin
The acute effects of local muscle vibration frequency on peak torque, rate of torque development, and EMG activity Derek N. Pamukoff, Eric D. Ryan, J. Troy Blackburn ⇑ Neuromuscular Research Laboratory, University of North Carolina at Chapel Hill, USA Department of Exercise and Sport Science, University of North Carolina at Chapel Hill, USA Curriculum in Human Movement Science, University of North Carolina at Chapel Hill, USA
a r t i c l e
i n f o
Article history: Received 7 December 2013 Received in revised form 9 July 2014 Accepted 28 July 2014
Keywords: Vibration Electromyography Quadriceps Neuromuscular
a b s t r a c t Purpose: Vibratory stimuli enhance muscle activity and may be used for rehabilitation and performance enhancement. Efficacy of vibration varies with the frequency of stimulation, but the optimal frequency is unclear. The purpose of this study was to examine the effects of 30 Hz and 60 Hz local muscle vibration (LMV) on quadriceps function. Methods: Twenty healthy volunteers (age = 20.4 ± 1.4 years, mass = 68.1 ± 11.0 kg, height = 170.1 ± 8.8 cm, males = 9) participated. Isometric knee extensor peak torque (PT), rate of torque development (RTD), and electromyography (EMG) of the quadriceps were assessed followed by one of the three LMV treatments (30 Hz, 60 Hz, control) applied under voluntary contraction, and again immediately, 5, 15, and 30 min post-treatment in three counterbalanced sessions. Dependent variables were analyzed using condition by time repeated-measures ANOVA. Results: The condition time interaction was significant for EMG amplitude (p = 0.001), but not for PT (p = 0.324) or RTD (p = 0.425). The increase in EMG amplitude following 30 Hz LMV was significantly greater than 60 Hz LMV and control. Conclusions: These findings suggest that 30 Hz LMV may elicit an improvement in quadriceps activation and could be used to treat quadriceps dysfunction resulting from knee pathologies. Ó 2014 Elsevier Ltd. All rights reserved.
1. Introduction Vibratory stimuli may have practical uses in exercise performance and rehabilitation. Early work suggests enhancement of reflexive activity via stimulation of muscle spindles causing the tonic vibratory reflex (Burke et al., 1976; Eklund and Hagbarth, 1966a,b). Other mechanisms of improved muscle function following vibration include elevated muscle temperature (Cochrane et al., 2008) and enhanced corticospinal excitability and intracortical processes (Mileva et al., 2009; Siggelkow et al., 1999). Despite studies reporting enhanced muscle function following vibration (Bosco et al., 1999; Tihanyi et al., 2007) there are also studies that report detrimental or equivocal (de Ruiter et al., 2003; Herda et al., 2009) effects. These ambiguous findings could be the result of heterogeneous stimulation parameters, particularly frequency. Greater damping of the stimulus occurs when the vibration frequency is close to ⇑ Corresponding author at: University of North Carolina at Chapel Hill, 124 Fetzer Hall, CB 8700, Chapel Hill, NC 27599-8700, USA. E-mail address: [email protected] (J. Troy Blackburn). http://dx.doi.org/10.1016/j.jelekin.2014.07.014 1050-6411/Ó 2014 Elsevier Ltd. All rights reserved.
the natural frequency of soft tissue (10–50 Hz in lower extremity musculature) (Wakeling and Nigg, 2001). Therefore, a muscle’s electrical and mechanical responses could vary with frequency of vibration. Greater gains have been reported in one-repetition-maximum during a half squat following whole body vibration (WBV) at 50 Hz, but not at 20 Hz or 35 Hz (Ronnestad, 2009). However, Moran et al. (2007) reported no improvement in peak power or EMG during a maximal biceps curl following 65 Hz LMV. Additionally, greater EMG amplitudes in the vastus lateralis have been observed during 30 Hz WBV compared to 40 Hz and 50 Hz (Cardinale and Lim, 2003). Overall, it remains unclear what frequency is ideal for enhancing muscle function. Much of the current literature has evaluated the effects of indirect vibration on muscle function using WBV. However, commercially available WBV platforms are cost prohibitive and provide limited portability. Local muscle vibration (LMV) applied directly to the muscle–tendon unit also influences muscle function (Bongiovanni and Hagbarth, 1990; Couto et al., 2013; Iodice et al., 2011; Mischi and Cardinale, 2009; Ribot-Ciscar et al., 2003), and may provide a cost effective and portable alternative to WBV. For example, Couto et al. (2013), found that maximal
D.N. Pamukoff et al. / Journal of Electromyography and Kinesiology 24 (2014) 888–894
voluntary contraction of the quadriceps improved after 4 weeks of 8 Hz and 26 Hz LMV. However, other studies indicate that LMV causes a reduction in force output (Kouzaki et al., 2000; Mottram et al., 2006), and this could be related to the parameters of stimulation. Also, the neurophysiological influences may differ between LMV and WBV, as WBV stimulates multiple receptors throughout the lower extremity (Pollock et al., 2011) and influence motor unit firing frequency and synchronization, muscle tuning, intramuscular coordination, and central motor command (Cochrane, 2011). However, LMV’s effects are likely restricted to receptors in the proximity of the stimulator, and are the result of neurogenic potentiation via the tonic vibratory reflex from stimulation of the muscle spindle system (Cardinale and Bosco, 2003). The efficacy LMV and WBV may differ due to differential damping characteristics. During WBV, the vibratory stimulus is damped by the musculature surrounding the ankle and knee joints, which may influence the magnitude of the stimulus delivered to the quadriceps and its neuromuscular response (Abercromby et al., 2007). Reduction in energy from the vibration signal may be minimized if the stimulus is delivered directly to the muscle via LMV. Therefore, the optimal frequency of stimulation may vary by method of delivery, and as far as we know, there are few studies that have evaluated the efficacy of different LMV frequencies on muscle function. Furthermore, it is unclear how long effects last following treatment, and studies are needed to determine if treatment effects persist following cessation of the stimulus. The purpose of this study was to compare the acute effects of 30 Hz vs. 60 Hz LMV exposure on quadriceps strength, rate of torque development, and EMG amplitude. We hypothesized that we would observe greater increases in quadriceps function following 30 Hz LMV compared to 60 Hz LMV and a control condition. A secondary aim was to determine the duration of the observed effects following LMV exposure. We hypothesized that the effects of LMV would be observed for up to 5 min following LMV as reported in previous studies (Bazett-Jones et al., 2008; McBride et al., 2010). 2. Materials and methods 2.1. Subjects An a priori power analysis (f = 0.4, alpha = 0.05, power = 0.8) suggested that 18 subjects would be needed to detect a significant difference in EMG activity between frequencies. Therefore, 20 healthy individuals (age = 20.4 ± 1.4 years, mass = 68.1 ± 11.0 kg, height = 170.1 ± 8.8 cm, males = 9) were recruited for participation. To be eligible for participation, subjects were required to be 18– 30 years of age and recreationally active, defined as participating in at least 30 min of physical activity 3 times per week. Individuals were excluded for known neurological disorders, lower extremity injuries within the past 6 months, history of lower extremity surgery, or medical conditions such as diabetes mellitus, high blood pressure, or cardiovascular disease. All subjects provided written informed consent prior to participation, and the study was approved by the university’s institutional review board.
889
the first session only, subjects received instruction of maximal voluntary isometric contraction (MVIC) procedures, and demonstration of LMV application. Next, subjects completed a baseline evaluation of peak torque (PT), rate of torque development (RTD), and surface EMG [vastus lateralis (VL), vastus medialis (VM), and rectus femoris (RF)] during a 5 s knee extension MVIC. Subjects then received one of the aforementioned interventions, and quadriceps MVICs were assessed immediately, 5 min, 15 min, and 30 min post-intervention. All data were sampled from the right leg (Fig. 1). The right leg was selected arbitrarily, but the within subjects design negates any bias of limb dominance. 2.3. Electromyography Electrode sites were shaved (where appropriate), lightly abraded, and cleaned with alcohol to enhance signal quality. Preamplified surface electrodes (EL254S, Biopac Systems, Baton Santa Barbara, CA) were placed as follows: VL – one third the distance along a line from the superolateral patella to the anterior superior iliac spine (ASIS), RF – half the distance from the ASIS to the center of the patella, VM – 80% on the line between the ASIS and the joint space in front of the anterior border of the medial collateral ligament (Fig. 1). All electrodes were placed longitudinally over the muscle, and a reference electrode was placed on the tibial tuberosity. EMG cables and electrodes were secured using prewrap and athletic tape to minimize motion artifact. 2.4. MVIC procedures Subjects were positioned in an isokinetic dynamometer with the knee in 60° of flexion as reported previously (Ritzmann et al., 2013). Subjects were instructed to extend their knee ‘‘as fast and hard as possible’’ in response to a visual stimulus. Subjects received verbal encouragement for all trials to ensure a maximal effort. Two trials were recorded at each time point and averaged for analysis. One minute of rest was provided between trials. 2.5. Local muscle vibration intervention A custom-built LMV device was secured over the quadriceps tendon (Fig. 1). Subjects were positioned in an isometric squat at approximately 40° of knee flexion using a handheld goniometer, and LMV was applied as 6 1 min treatment (2 min of rest between exposures) at a frequency of 30 Hz (displacement = 1.6 mm) or 60 Hz (displacement = 0.4 mm). Displacement varied between conditions to permit constant acceleration of the stimulus (2g) according to the equation: apeak = 2 p2 f2 D (where apeak is peak acceleration, f is frequency, D is displacement, (Rauch et al., 2010)). These parameters were selected based on pilot data and those reported in previous literature (Tihanyi et al., 2007). During the control condition, subjects were placed in the same position but received no vibration. The squat position was selected to increase tension in the quadriceps and improve vibration transmission (Burke and Gandevia, 1995). 2.6. Signal processing
2.2. Experimental design A single-group, repeated measures, crossover design was used to evaluate the influence of 2 LMV frequencies on neuromuscular activity of the quadriceps. Data collection occurred during 3 testing sessions (30 Hz, 60 Hz, control) separated by one week intervals. The order of testing conditions was counterbalanced using a Latin square to control for confounding effects such as familiarization and fatigue. During each testing session, subjects completed a 5min warm-up on a cycle ergometer at a self-selected pace. During
EMG and torque data were sampled simultaneously at 2 kHz using the Biopac data acquisition system (MP150WSW, Biopac Systems Inc., Santa Barbara, CA). Raw EMG signals were corrected for DC bias, bandpass filtered using a 4th order zero-phase lag Butterworth filter (20–350 Hz) and notch filtered (59.5–60.5 Hz). The filtered data were smoothed using a 20 ms root-mean-square (RMS) sliding window function. EMG amplitude was calculated as the mean amplitude during the MVIC (RMSavg). The baseline amplitude values from each respective session were used as a standardization
890
D.N. Pamukoff et al. / Journal of Electromyography and Kinesiology 24 (2014) 888–894
Fig. 1. Top left – EMG sensor placement, top right – MVIC testing position, bottom left – LMV treatment, bottom right – LMV device (the LMV stimulator consisted of a singleaxis electromagnetic oscillator mounted on a plastic frame. The under surface of the frame was curved to accommodate the shape of the anterior thigh. The device was coupled with an amplifier causing it to vibrate in the anterior–posterior direction. Frequency was held constant at 30 Hz (displacement = 1.6 mm) or 60 Hz (displacement = 0.6 mm), and the acceleration was constrained to 2g via feedback provided to the controller unit from an accelerometer.
criterion, and the percent of baseline amplitude was calculated [(Followupamp/Baselineamp) 100] for each muscle and averaged across the VL, VM, and RF to create a composite measure of muscle activity for analysis. Torque data were low pass filtered at 50 Hz (4th order Butterworth), and peak torque (PT) and rate of torque development (RTD) were calculated from the resulting torque vs. time curve. PT was defined as the maximal voluntary torque value and was normalized to body mass for analysis (PTnorm). Change in torque (final–initial) was calculated over successive 20 ms intervals and divided by time to represent RTD. The peak value was identified and normalized to body mass for statistical analyses (RTDnorm). 2.7. Statistical analyses All data were confirmed as being normality distributed via the Shapiro–Wilk test and homogeneity of variance using Levene’s test, thus they met the assumptions for analysis of variance (ANOVA). Intrasession reliability (ICC3,1) was calculated using the baseline trials of the control session, and intersession reliability (ICC2,k) was calculated using the baseline trials of each condition. Dependent variables (PTnorm, RTDnorm, RMSavg) were compared between conditions at baseline using one-way repeated measures
ANOVA. 3 5 (condition time) ANOVAs were used to analyze PTnorm, RTDnorm, and RMSavg between conditions from pretest to each posttest (immediately, 5 min, 15 min, and 30 min following treatment). The level of significance was set to a 6 0.05. Evaluation of the 95% confidence intervals of the mean difference between pairs was used for post hoc analyses, with intervals not crossing 0 considered significant. 3. Results Data were screened for outliers and none were identified, therefore all subjects were used for analysis. All data were treated as normal, and the ICC values ranged from 0.70 to 0.90 (good to excellent, Table 1). Baseline values for the dependent variables did not differ between conditions (Table 2). 3.1. Electromyography A significant condition by time interaction was observed for the RMSavg of the quadriceps (p = 0.001). Post hoc analyses (Table 3) indicated an increase in RMSavg in the 30 Hz condition only from baseline to immediately following treatment (+11.55%, 95%CI: 6.07–17.13). RMSavg was greater in the 30 Hz compared to the
891
D.N. Pamukoff et al. / Journal of Electromyography and Kinesiology 24 (2014) 888–894 Table 1 Reliability analyses.
Peak Torque 30Hz
Measure
Intrasession reliability (ICC3,1)
Intersession reliability (ICC2,k)
VMRMS RFRMS VLRMS PTnorm RTDnorm
0.941 0.948 0.919 0.981 0.741
0.863 0.754 0.898 0.892 0.704
60Hz
Control
3.50
Table 2 One way ANOVA of baseline values. Variable
30 Hz
60 Hz
Control
p
PTnorm (N m/kg) RTDnorm (N m/s/kg) RMSavg (mV)
2.7 (0.7) 20.3 (7.0) 0.59 (0.029)
2.7 (0.8) 20.6 (8.9) 0.59 (0.028)
2.6 (0.8) 19.8 (6.7) 0.61 (0.029)
0.612 0.632 0.809
Peak Toruqe (Nm/kg)
3.00
2.50
2.00
1.50
1.00
Pre
60 Hz (+6.77%, 95%CI: 2.08–11.52) and control condition (+15.20%, 95%CI: 8.63–21.77) immediately following treatment. RMSavg was also greater in the 60 Hz compared to the control condition (+8.43%, 95%CI: 1.29–15.51) immediately following treatment. The increase in RMSavg was evident 5 min following treatment in the 30 Hz condition only (+6.06%, 95%CI: 0.76–11.44), and was greater than the 60 Hz (+7.31%, 95%CI: 1.08–13.52) and control condition (+8.43%, 95%CI: 1.29–15.51). There were no significant differences in RMSavg from baseline to 15 or 30 min following treatment in any condition, and RMSavg did not differ between conditions at 15 or 30 min following treatment.
3.2. Peak torque and rate of torque development The condition by time interaction was not significant for PTnorm (p = 0.325, Fig. 2) or RTDnorm (p = 0.425, Fig. 3). Given that there
Post
5 min Post
15 min Post
30 min Post
Fig. 2. Normalized peak torque.
was a significant interaction in the RMSavg, we expected to see an associated increase in PTnorm. Additionally, the change in RMSavg and change in PTnorm from baseline to immediately post treatment were positively correlated (r = 0.55, p = 0.016). Therefore, we performed exploratory post hoc analyses of the interaction effect for PTnorm since visual inspection of the data suggested differential effects of the LMV frequencies. The ANOVA model had a power of 0.65 (f = 0.37), indicating that we may have been underpowered to detect differences in PTnorm, and 7 additional subjects would be necessary to achieve a power of 0.8. However, there was a significant increase in PTnorm in the 30 Hz condition of 0.13 ± 0.61 N m/kg (p < 0.001) from baseline to immediately following the intervention but not in the 60 Hz ( 0.02 ± 0.15 N m/kg, p = 0.54) or control ( 0.02 ± 0.25 N m/kg, p = 0.76) conditions.
Table 3 Mean RMS of the EMG signal. Comparison
95% confidence interval
p
Lower
Upper
+11.55 +4.78 3.65 +15.20 +8.43 +6.77
6.07 1.27 8.65 8.63 1.29 1.05
17.13 10.87 1.25 21.77 15.51 12.49
0.001 0.095 0.109
Contents lists available at ScienceDirect
Journal of Electromyography and Kinesiology journal homepage: www.elsevier.com/locate/jelekin
The acute effects of local muscle vibration frequency on peak torque, rate of torque development, and EMG activity Derek N. Pamukoff, Eric D. Ryan, J. Troy Blackburn ⇑ Neuromuscular Research Laboratory, University of North Carolina at Chapel Hill, USA Department of Exercise and Sport Science, University of North Carolina at Chapel Hill, USA Curriculum in Human Movement Science, University of North Carolina at Chapel Hill, USA
a r t i c l e
i n f o
Article history: Received 7 December 2013 Received in revised form 9 July 2014 Accepted 28 July 2014
Keywords: Vibration Electromyography Quadriceps Neuromuscular
a b s t r a c t Purpose: Vibratory stimuli enhance muscle activity and may be used for rehabilitation and performance enhancement. Efficacy of vibration varies with the frequency of stimulation, but the optimal frequency is unclear. The purpose of this study was to examine the effects of 30 Hz and 60 Hz local muscle vibration (LMV) on quadriceps function. Methods: Twenty healthy volunteers (age = 20.4 ± 1.4 years, mass = 68.1 ± 11.0 kg, height = 170.1 ± 8.8 cm, males = 9) participated. Isometric knee extensor peak torque (PT), rate of torque development (RTD), and electromyography (EMG) of the quadriceps were assessed followed by one of the three LMV treatments (30 Hz, 60 Hz, control) applied under voluntary contraction, and again immediately, 5, 15, and 30 min post-treatment in three counterbalanced sessions. Dependent variables were analyzed using condition by time repeated-measures ANOVA. Results: The condition time interaction was significant for EMG amplitude (p = 0.001), but not for PT (p = 0.324) or RTD (p = 0.425). The increase in EMG amplitude following 30 Hz LMV was significantly greater than 60 Hz LMV and control. Conclusions: These findings suggest that 30 Hz LMV may elicit an improvement in quadriceps activation and could be used to treat quadriceps dysfunction resulting from knee pathologies. Ó 2014 Elsevier Ltd. All rights reserved.
1. Introduction Vibratory stimuli may have practical uses in exercise performance and rehabilitation. Early work suggests enhancement of reflexive activity via stimulation of muscle spindles causing the tonic vibratory reflex (Burke et al., 1976; Eklund and Hagbarth, 1966a,b). Other mechanisms of improved muscle function following vibration include elevated muscle temperature (Cochrane et al., 2008) and enhanced corticospinal excitability and intracortical processes (Mileva et al., 2009; Siggelkow et al., 1999). Despite studies reporting enhanced muscle function following vibration (Bosco et al., 1999; Tihanyi et al., 2007) there are also studies that report detrimental or equivocal (de Ruiter et al., 2003; Herda et al., 2009) effects. These ambiguous findings could be the result of heterogeneous stimulation parameters, particularly frequency. Greater damping of the stimulus occurs when the vibration frequency is close to ⇑ Corresponding author at: University of North Carolina at Chapel Hill, 124 Fetzer Hall, CB 8700, Chapel Hill, NC 27599-8700, USA. E-mail address: [email protected] (J. Troy Blackburn). http://dx.doi.org/10.1016/j.jelekin.2014.07.014 1050-6411/Ó 2014 Elsevier Ltd. All rights reserved.
the natural frequency of soft tissue (10–50 Hz in lower extremity musculature) (Wakeling and Nigg, 2001). Therefore, a muscle’s electrical and mechanical responses could vary with frequency of vibration. Greater gains have been reported in one-repetition-maximum during a half squat following whole body vibration (WBV) at 50 Hz, but not at 20 Hz or 35 Hz (Ronnestad, 2009). However, Moran et al. (2007) reported no improvement in peak power or EMG during a maximal biceps curl following 65 Hz LMV. Additionally, greater EMG amplitudes in the vastus lateralis have been observed during 30 Hz WBV compared to 40 Hz and 50 Hz (Cardinale and Lim, 2003). Overall, it remains unclear what frequency is ideal for enhancing muscle function. Much of the current literature has evaluated the effects of indirect vibration on muscle function using WBV. However, commercially available WBV platforms are cost prohibitive and provide limited portability. Local muscle vibration (LMV) applied directly to the muscle–tendon unit also influences muscle function (Bongiovanni and Hagbarth, 1990; Couto et al., 2013; Iodice et al., 2011; Mischi and Cardinale, 2009; Ribot-Ciscar et al., 2003), and may provide a cost effective and portable alternative to WBV. For example, Couto et al. (2013), found that maximal
D.N. Pamukoff et al. / Journal of Electromyography and Kinesiology 24 (2014) 888–894
voluntary contraction of the quadriceps improved after 4 weeks of 8 Hz and 26 Hz LMV. However, other studies indicate that LMV causes a reduction in force output (Kouzaki et al., 2000; Mottram et al., 2006), and this could be related to the parameters of stimulation. Also, the neurophysiological influences may differ between LMV and WBV, as WBV stimulates multiple receptors throughout the lower extremity (Pollock et al., 2011) and influence motor unit firing frequency and synchronization, muscle tuning, intramuscular coordination, and central motor command (Cochrane, 2011). However, LMV’s effects are likely restricted to receptors in the proximity of the stimulator, and are the result of neurogenic potentiation via the tonic vibratory reflex from stimulation of the muscle spindle system (Cardinale and Bosco, 2003). The efficacy LMV and WBV may differ due to differential damping characteristics. During WBV, the vibratory stimulus is damped by the musculature surrounding the ankle and knee joints, which may influence the magnitude of the stimulus delivered to the quadriceps and its neuromuscular response (Abercromby et al., 2007). Reduction in energy from the vibration signal may be minimized if the stimulus is delivered directly to the muscle via LMV. Therefore, the optimal frequency of stimulation may vary by method of delivery, and as far as we know, there are few studies that have evaluated the efficacy of different LMV frequencies on muscle function. Furthermore, it is unclear how long effects last following treatment, and studies are needed to determine if treatment effects persist following cessation of the stimulus. The purpose of this study was to compare the acute effects of 30 Hz vs. 60 Hz LMV exposure on quadriceps strength, rate of torque development, and EMG amplitude. We hypothesized that we would observe greater increases in quadriceps function following 30 Hz LMV compared to 60 Hz LMV and a control condition. A secondary aim was to determine the duration of the observed effects following LMV exposure. We hypothesized that the effects of LMV would be observed for up to 5 min following LMV as reported in previous studies (Bazett-Jones et al., 2008; McBride et al., 2010). 2. Materials and methods 2.1. Subjects An a priori power analysis (f = 0.4, alpha = 0.05, power = 0.8) suggested that 18 subjects would be needed to detect a significant difference in EMG activity between frequencies. Therefore, 20 healthy individuals (age = 20.4 ± 1.4 years, mass = 68.1 ± 11.0 kg, height = 170.1 ± 8.8 cm, males = 9) were recruited for participation. To be eligible for participation, subjects were required to be 18– 30 years of age and recreationally active, defined as participating in at least 30 min of physical activity 3 times per week. Individuals were excluded for known neurological disorders, lower extremity injuries within the past 6 months, history of lower extremity surgery, or medical conditions such as diabetes mellitus, high blood pressure, or cardiovascular disease. All subjects provided written informed consent prior to participation, and the study was approved by the university’s institutional review board.
889
the first session only, subjects received instruction of maximal voluntary isometric contraction (MVIC) procedures, and demonstration of LMV application. Next, subjects completed a baseline evaluation of peak torque (PT), rate of torque development (RTD), and surface EMG [vastus lateralis (VL), vastus medialis (VM), and rectus femoris (RF)] during a 5 s knee extension MVIC. Subjects then received one of the aforementioned interventions, and quadriceps MVICs were assessed immediately, 5 min, 15 min, and 30 min post-intervention. All data were sampled from the right leg (Fig. 1). The right leg was selected arbitrarily, but the within subjects design negates any bias of limb dominance. 2.3. Electromyography Electrode sites were shaved (where appropriate), lightly abraded, and cleaned with alcohol to enhance signal quality. Preamplified surface electrodes (EL254S, Biopac Systems, Baton Santa Barbara, CA) were placed as follows: VL – one third the distance along a line from the superolateral patella to the anterior superior iliac spine (ASIS), RF – half the distance from the ASIS to the center of the patella, VM – 80% on the line between the ASIS and the joint space in front of the anterior border of the medial collateral ligament (Fig. 1). All electrodes were placed longitudinally over the muscle, and a reference electrode was placed on the tibial tuberosity. EMG cables and electrodes were secured using prewrap and athletic tape to minimize motion artifact. 2.4. MVIC procedures Subjects were positioned in an isokinetic dynamometer with the knee in 60° of flexion as reported previously (Ritzmann et al., 2013). Subjects were instructed to extend their knee ‘‘as fast and hard as possible’’ in response to a visual stimulus. Subjects received verbal encouragement for all trials to ensure a maximal effort. Two trials were recorded at each time point and averaged for analysis. One minute of rest was provided between trials. 2.5. Local muscle vibration intervention A custom-built LMV device was secured over the quadriceps tendon (Fig. 1). Subjects were positioned in an isometric squat at approximately 40° of knee flexion using a handheld goniometer, and LMV was applied as 6 1 min treatment (2 min of rest between exposures) at a frequency of 30 Hz (displacement = 1.6 mm) or 60 Hz (displacement = 0.4 mm). Displacement varied between conditions to permit constant acceleration of the stimulus (2g) according to the equation: apeak = 2 p2 f2 D (where apeak is peak acceleration, f is frequency, D is displacement, (Rauch et al., 2010)). These parameters were selected based on pilot data and those reported in previous literature (Tihanyi et al., 2007). During the control condition, subjects were placed in the same position but received no vibration. The squat position was selected to increase tension in the quadriceps and improve vibration transmission (Burke and Gandevia, 1995). 2.6. Signal processing
2.2. Experimental design A single-group, repeated measures, crossover design was used to evaluate the influence of 2 LMV frequencies on neuromuscular activity of the quadriceps. Data collection occurred during 3 testing sessions (30 Hz, 60 Hz, control) separated by one week intervals. The order of testing conditions was counterbalanced using a Latin square to control for confounding effects such as familiarization and fatigue. During each testing session, subjects completed a 5min warm-up on a cycle ergometer at a self-selected pace. During
EMG and torque data were sampled simultaneously at 2 kHz using the Biopac data acquisition system (MP150WSW, Biopac Systems Inc., Santa Barbara, CA). Raw EMG signals were corrected for DC bias, bandpass filtered using a 4th order zero-phase lag Butterworth filter (20–350 Hz) and notch filtered (59.5–60.5 Hz). The filtered data were smoothed using a 20 ms root-mean-square (RMS) sliding window function. EMG amplitude was calculated as the mean amplitude during the MVIC (RMSavg). The baseline amplitude values from each respective session were used as a standardization
890
D.N. Pamukoff et al. / Journal of Electromyography and Kinesiology 24 (2014) 888–894
Fig. 1. Top left – EMG sensor placement, top right – MVIC testing position, bottom left – LMV treatment, bottom right – LMV device (the LMV stimulator consisted of a singleaxis electromagnetic oscillator mounted on a plastic frame. The under surface of the frame was curved to accommodate the shape of the anterior thigh. The device was coupled with an amplifier causing it to vibrate in the anterior–posterior direction. Frequency was held constant at 30 Hz (displacement = 1.6 mm) or 60 Hz (displacement = 0.6 mm), and the acceleration was constrained to 2g via feedback provided to the controller unit from an accelerometer.
criterion, and the percent of baseline amplitude was calculated [(Followupamp/Baselineamp) 100] for each muscle and averaged across the VL, VM, and RF to create a composite measure of muscle activity for analysis. Torque data were low pass filtered at 50 Hz (4th order Butterworth), and peak torque (PT) and rate of torque development (RTD) were calculated from the resulting torque vs. time curve. PT was defined as the maximal voluntary torque value and was normalized to body mass for analysis (PTnorm). Change in torque (final–initial) was calculated over successive 20 ms intervals and divided by time to represent RTD. The peak value was identified and normalized to body mass for statistical analyses (RTDnorm). 2.7. Statistical analyses All data were confirmed as being normality distributed via the Shapiro–Wilk test and homogeneity of variance using Levene’s test, thus they met the assumptions for analysis of variance (ANOVA). Intrasession reliability (ICC3,1) was calculated using the baseline trials of the control session, and intersession reliability (ICC2,k) was calculated using the baseline trials of each condition. Dependent variables (PTnorm, RTDnorm, RMSavg) were compared between conditions at baseline using one-way repeated measures
ANOVA. 3 5 (condition time) ANOVAs were used to analyze PTnorm, RTDnorm, and RMSavg between conditions from pretest to each posttest (immediately, 5 min, 15 min, and 30 min following treatment). The level of significance was set to a 6 0.05. Evaluation of the 95% confidence intervals of the mean difference between pairs was used for post hoc analyses, with intervals not crossing 0 considered significant. 3. Results Data were screened for outliers and none were identified, therefore all subjects were used for analysis. All data were treated as normal, and the ICC values ranged from 0.70 to 0.90 (good to excellent, Table 1). Baseline values for the dependent variables did not differ between conditions (Table 2). 3.1. Electromyography A significant condition by time interaction was observed for the RMSavg of the quadriceps (p = 0.001). Post hoc analyses (Table 3) indicated an increase in RMSavg in the 30 Hz condition only from baseline to immediately following treatment (+11.55%, 95%CI: 6.07–17.13). RMSavg was greater in the 30 Hz compared to the
891
D.N. Pamukoff et al. / Journal of Electromyography and Kinesiology 24 (2014) 888–894 Table 1 Reliability analyses.
Peak Torque 30Hz
Measure
Intrasession reliability (ICC3,1)
Intersession reliability (ICC2,k)
VMRMS RFRMS VLRMS PTnorm RTDnorm
0.941 0.948 0.919 0.981 0.741
0.863 0.754 0.898 0.892 0.704
60Hz
Control
3.50
Table 2 One way ANOVA of baseline values. Variable
30 Hz
60 Hz
Control
p
PTnorm (N m/kg) RTDnorm (N m/s/kg) RMSavg (mV)
2.7 (0.7) 20.3 (7.0) 0.59 (0.029)
2.7 (0.8) 20.6 (8.9) 0.59 (0.028)
2.6 (0.8) 19.8 (6.7) 0.61 (0.029)
0.612 0.632 0.809
Peak Toruqe (Nm/kg)
3.00
2.50
2.00
1.50
1.00
Pre
60 Hz (+6.77%, 95%CI: 2.08–11.52) and control condition (+15.20%, 95%CI: 8.63–21.77) immediately following treatment. RMSavg was also greater in the 60 Hz compared to the control condition (+8.43%, 95%CI: 1.29–15.51) immediately following treatment. The increase in RMSavg was evident 5 min following treatment in the 30 Hz condition only (+6.06%, 95%CI: 0.76–11.44), and was greater than the 60 Hz (+7.31%, 95%CI: 1.08–13.52) and control condition (+8.43%, 95%CI: 1.29–15.51). There were no significant differences in RMSavg from baseline to 15 or 30 min following treatment in any condition, and RMSavg did not differ between conditions at 15 or 30 min following treatment.
3.2. Peak torque and rate of torque development The condition by time interaction was not significant for PTnorm (p = 0.325, Fig. 2) or RTDnorm (p = 0.425, Fig. 3). Given that there
Post
5 min Post
15 min Post
30 min Post
Fig. 2. Normalized peak torque.
was a significant interaction in the RMSavg, we expected to see an associated increase in PTnorm. Additionally, the change in RMSavg and change in PTnorm from baseline to immediately post treatment were positively correlated (r = 0.55, p = 0.016). Therefore, we performed exploratory post hoc analyses of the interaction effect for PTnorm since visual inspection of the data suggested differential effects of the LMV frequencies. The ANOVA model had a power of 0.65 (f = 0.37), indicating that we may have been underpowered to detect differences in PTnorm, and 7 additional subjects would be necessary to achieve a power of 0.8. However, there was a significant increase in PTnorm in the 30 Hz condition of 0.13 ± 0.61 N m/kg (p < 0.001) from baseline to immediately following the intervention but not in the 60 Hz ( 0.02 ± 0.15 N m/kg, p = 0.54) or control ( 0.02 ± 0.25 N m/kg, p = 0.76) conditions.
Table 3 Mean RMS of the EMG signal. Comparison
95% confidence interval
p
Lower
Upper
+11.55 +4.78 3.65 +15.20 +8.43 +6.77
6.07 1.27 8.65 8.63 1.29 1.05
17.13 10.87 1.25 21.77 15.51 12.49
0.001 0.095 0.109
